The present invention relates to a gasket and a medical device using the same.
Known are medical devices such as disposable injectors each of which comprises a cylindrical body and a gasket which can move in close contact with the inner surface of the cylindrical body.
Conventionally, the above-described gasket is made of, as disclosed in Japanese Patent Publication No. 59-18427 U.S. Pat. No. 4,303,070, vulcanized rubber or styrene type thermoplastic elastomer (to be called "TPE" hereinafter). The styrene type TPE is made of a composite consisting of hydrogen-added derivative of styrene-butadiene block copolymer, plasticizer, and polypropylene for improving workability and for adjusting the hardness.
A large facility is needed to manufacture the gasket made of vulcanized rubber since such a gasket is manufactured after it has been subjected to a complicated vulcanization process. Furthermore, a variety of additives such as sulfur, a vulcanization accelerator, and a filler are necessary. As a result, there is a fear of an elution of the additives into the liquid medicine when the medical device is used.
On the contrary, although the gasket made of the styrene TPE exhibits an advantage in the formability at the time of the injection molding and another advantage in that the fear of elution of sulfur can be eliminated, it is disadvantageous relative to vulcanized rubber in terms of the elasticity of the rubber, where the term "the elasticity of rubber" means in particular the tensile characteristics whereby that the inflection points such as the yielding points are reduced much as possible and also the modulus (for example, the stress when expanded by 300%) is at a relatively high level in relation to the hardness. That is, in the styrene TPE, polypropylene which is the component which causes the inflection point such as the yield point to be generated also causes the modulus to be lowered in the tensile characteristics. Therefore, in a case where the gasket made of the styrene TPE is employed in an injector, the displacement added to a portion to be pushed does not immediately correspond to the distance of movement of the gasket. Therefore, a problem arises in that the gasket intermittently moves; that is, a so-called pulsation phenomenon takes place.
An object of the present invention is to provide a gasket and a medical device using the same, the gasket exhibiting an excellent formability and safety against elution and being capable of preventing pulsation phenomenon.